In the oil industry, it is usual practice to measure a continuous record of the borehole size so that for example variation in the borehole diameter can be further analyzed. As the result of such analysis, it is possible to detect the presence of fractures in the wall of the borehole or the presence of soft, non-competent rock.
In many instances, a mechanical caliper tool is lowered into the well to make a measurement of the borehole dimensions. For example, Schlumberger's PPC™ tool, also known as the Powered Positioning Caliper, uses four independently powered arms to perform four independent measurements of the distance between the tool body and the wall of the borehole. These measurements allow calculations of the long and short axis diameters of elliptical boreholes.
As documented for example by the co-owned U.S. Pat. No. 6,891,777 to Pabon and Sloan and the prior art cited therein, alternative methods of measuring the borehole diameter make use of acoustic or, more specifically, ultrasonic waves. These methods are mostly based on determining the travel time between the emission of a short pulse of acoustic signal and the arrival of its echo as reflected from the wall of the borehole. Using knowledge of the speed of sound in the fluid filling the borehole it is possible to covert time measurements into distances.
The known acoustic caliper measurements tend to use ultrasonics in the range of 200 kHz to 2 MHz as a shorter wavelength results in more accurate determination of the arrival times and hence the borehole dimensions. In most tool designs it is further required to keep the pulse length very short to avoid tool movements interfering with the measurements. Thus pulse duration can be in the order of 10 milliseconds or even shorter.
Apart from caliper measurements it is also well known to deploy acoustic logging tools such Schlumberger's DSI™ or Sonic Scanner™ in wells to determine the acoustic properties of the formation surrounding the wells. A state-of-the-art acoustic logging tool such as the Sonic Scanner uses one or more sources and several receivers mounted along the body of the tool. These sources generate wave modes in the borehole. Depending on whether the source is a monopole source or a dipole source, the waves generated are typically either the lowest order axisymmetric wave mode, referred to as Stoneley wave mode, or lowest-order flexural wave modes. The operational bandwidth of sources for acoustic logging is generally in the frequency range of 0.5 to 20 kHz, with the Stoneley mode typically observed in the 0.5 to 5 kHz frequency range.
The waves generated in the borehole by acoustic logging tools cause compressional and shear waves to propagate in the formation, which in turn, radiate energy back into the borehole. The receivers are used to measure the pressure waves of the energy coming back into the borehole, and also those waves which travel directly along the borehole, such as the above-mentioned Stoneley wave. With these measurements, it is possible to determine the velocities or slownesses of the compressional or shear wave and some of the elastic constants or moduli of the rock in the formation surrounding the borehole, as described for example in published U.S. Patent Application Publication No. 2006/0256656 and by J. L. Arroyo Franco et al. in: “Sonic Investigations In and Around the Borehole”, Oilfield Review, Spring 2006, pp. 14-33.
The Stoneley wave mode of sonic logging tools is also routinely employed as an indicator for fractures in the formation surrounding the borehole. As detailed for example in co-owned U.S. Pat. No. 4,870,627 to K. Hsu et al. and in the more recent U.S. Pat. No. 6,192,316 to B. Hornby and the references cited therein, fractures in the wall of a borehole cause secondary Stoneley waves, which can be analyzed to determine the fracture location and width.
In published U.S. Patent Application Publication No. 2007/0104027, there is described a method of measuring the depth of a perforation tunnel using an acoustic transmitter and receiver arranged respectively below and above a perforation in the well.
In view of the known art, it is seen as one object of the invention to provide a method and apparatus for determining the cross-sectional area of a borehole and to extend the use of such method and apparatus to facilitate the detection of fractures that intersect the wall of the borehole.